The present invention relates to an optical recording medium, and more specifically to an optical disk which can record data in heat mode when irradiated with a laser beam.
The recording mechanism of conventional memory optical disks is classified into the three categories shown in FIGS. 1 to 3.
In the first recording mechanism, various types of information are recorded in the following manner. A thin film 2 of a low-melting point material, such as Te or a Te alloy, and having a thickness of 20 to 80 nm is formed on a transparent substrate 1 of glass, polyacrylic resin, polycarbonate resin or the like. This film is irradiated with a laser beam, and its irradiated portion melts or evaporates, thereby forming a small pit 3. Thus, information is recorded.
In the second recording mechanism shown in FIG. 2, a gas-releasing film 5 and a metal film 6 are formed on a substrate 4. When the film 6 is irradiated with a laser beam, a gas is released in the irradiated portion of gas releasing film 5 by heat generated upon laser beam irradiation. The gas forms voids which result in raised blisters or bubbles 7 in the metal film 6, thereby recording information.
In the third recording mechanism shown in FIG. 3, a thin film 9, which changes its structure upon temperature change, is formed on a substrate 8. The film 9 is irradiated with a laser beam. The irradiated portion has its structure changed. For example, this portion changes from a crystalline structure to an amorphous structure, thereby changing reflectance and recording information.
Another recording mechanism is known. As disclosed in Japanese Patent Publication No. 54-20137 and Appl. Phys. Lett. vol. 39, 927 (1981), a disk with recording and non-recording parts have different transmission/reflection characteristics is used. When any part is irradiated with a laser beam, its transmission/reflection characteristics is detected to determine whether information is recorded or not.
In the recording mechanisms shown in FIGS. 1 and 2, upon recording information, the recording film evaporates or changes its shape. Therefore, when a protective film is directly formed on the recording film, the recording sensitivity is degraded significantly. Therefore, to make practical use of these mechanisms, an air-sandwich structure must be adopted wherein the protective plate is arranged at a distance from the recording film. A disk of this complex structure is difficult to manufacture. In the recording mechanism shown in FIG. 3, even if the protective film is formed directly on the recording film, the recording sensitivity will not be significantly decreased. However, since this mechanism utilizes the structural change of the recording film, the boundary between recording and non-recording parts is unstable. Hence, the lifetime of the recorded data is short.